4.2-From Eohippus to Equus -Equine Evolution

The evolution of the horse has traditionally been based on three factors: overall size of the animal, limb structure and teeth (Sautter and Glover, 1981, Waring, 1983, Biracree and Insinger, 1982, Prothero and Schoch, 2002[1]). The basic characteristics of the main ancestral lines can be best illustrated in the following chart (See Table 4.2). For an illustration of the equine evolutionary tree, see Table 4.4 at the end of this section.

Table 4.2

Main lines of horse ancestry

Size

Limb structure

Teeth

Other characteristics

Eohippus or

Hyracotherium

10-20 inches at shoulder

Pads on toes

Browsers

No distinct muzzle

Mesohippus

24 inches at shoulder

Pads on toes

Browsers

Larger brain

Miohippus

28-32 inches at shoulder

Pads on toes

Browsers and Grazers

Cecal digestion

Parahippus

34-38 inches at shoulder

One functional toe, no pads

Cement protection

Longer muzzle

Merychippus or hipparion

40 inches at shoulder-varied with each species

Hipparion had facial fossa

Dinohippus

48-52 inches at shoulder-varies with each species

Exclusive grazers

Shallow fossa

Equus

60-72 inched at shoulder-varies with each species

Modern horse

Source: adapted from the text of Prothero and Schoch, 2002 and Waring 1983.

Fossils found in China throughout the 1980’s suggest that Radinskya is the oldest ancestor of the horse. It would have been found throughout Asia during the Paleocene epoch, which was 60 million years ago (Table 4.1). Radinskya must have crossed the Bering land-bridge just as the Eocene epoch was beginning, giving way to the Hyracotherium or Eohippus (known as Protorohippus in Figure 4.3) in America and the Old World. The sexes of this mammal had distinct anatomical differences. The males had a larger, domed skull filled with relatively large canine teeth, while the females had flatter skulls and smaller canine teeth. According to Prothero and Schoch (2002), this could be due to a combination of ecological and social issues. They believe it is likely that the vegetation of the early Eocene epoch (Table 4.1) resembled open parkland. The environment encouraged Hyracotherium to band in herds with one larger male protecting a group of females and their offspring. These mammals hardly appeared to be primitive horses. However, scientists are confident the subtle distinguishing marks in the teeth and inner-ear region give diagnostic proof that Hyracotherium is indeed an early ancestor of the horse.

Figure 4.1 -Drawing of Eohippus

Source: Prothero and Schoch, 2002

Hyracotherium eventually gave way to the “middle horses” of Mesohippus and Miohippus (Prothero and Schoch, 2002, 204).These mammals more closely resembled modern equines, although they stood just over two feet tall (Table 4.2).Previously thought to have been completely three-toed mammals, recent evidence suggests the change from four toes to three toes occurs within the Mesohippus evolution (Prothero and Schoch, 2002). Some Mesohippus fossils possess four functional toes, while others possess only three.Neil Shubin has done much work regarding the embryonic development of the toes in horses (Prothero and Schoch, 2002). He has discovered that while it is nearly impossible for a horse to gradually lose an inner toe (comparable to a human thumb) which is formed first, it takes only a slight mutation to stop the formation of the outermost toe (a human “pinky” finger) in any given individual horse.This explains why some Mesohippus had four toes and others had three.It also explains the presence of modern-day mutant horses born with partially developed side toes.

Figure 4.2 -Drawing of Mesohippus

Source: Prothero and Schoch, 2002

Older texts show Mesohippus as an unbranched evolutionary trunk immediately followed by an unbranched Miohippus trunk. Recent work in South Dakota, Wyoming and Nebraska has shown the trunks of both mammals are very “bushy” with many side branches developing modern horse characteristics independently of one another (Prothero and Schoch, 2002). Both lines lived in the Eocene and Oligocene epochs (Table 4.1), although only Miohippus survived into the late Oligocene. The main difference between the two genera is in the teeth. All horses up to this point had “low-crowned” teeth, which, like human teeth, have a well-defined root and crown and do not grow continuously. As Miohippus began to graze on a grittier diet in addition to browsing for seeds, fruits and leaves, it became necessary to have a tooth surface that would continuously replace itself as it wore down. The teeth of “high-crowned” mammals, such as Miohippus, are long prisms, with no distinct root and crown. The teeth protrude into the bottom of the jaw and up into the facial region, gradually filling in as the tooth surface is worn away. As Miohippus gave way to Parahippus in the Miocene epoch, a layer of cement could be found on the teeth which slowed the erosion of the tooth surface and gave even more advantage to the grazing animals.

Parahippus showed even more evidence that the transition from marshy forests to open grasslands took place. The muzzle and facial region grew larger to accommodate the higher-crowned and complex teeth. Parahippus adapted to life on open grasslands by developing longer legs that moved only to the front and back. Previous species were able to rotate their legs, which may have been necessary for negotiating marshy conditions. However, Parahippus’ primary defense from predators was to run, which requires only front and back movement of the legs. The radius became permanently attached to the ulna in the forelimbs, which prohibits any rotation. The hind limbs also changed as the tibia became the main weight bearing bone and the fibula became vestigial.

The late Miocene epoch (Table 4.1) witnessed the greatest equine diversification ever known (Prothero and Schoch, 2002). Paleontologists often group all equine species into the “wastebasket” (Prothero and Schoch, 2002, 208) Merychippus group since many of the species living in this time period were very similar to each other. However, it is now known through the work of Morris Skinner (Prothero and Schoch, 2002) that most of the species within the Merychippus genus are “hipparion” and are not ancestors of the modern horse. Instead, they are side branches of the evolutionary ladder that have long since been extinct. Five major groups of Merychippus lived in the late Miocene, three of which became “hipparions”. The last two groups, which are Protohippus-Calippus and Pliohippus, led to the truly one-toed genus Dinohippus.

Figure 4.3 -Drawing of Merychippus

Source: Prothero and Schoch, 2002

Dinohippus and Equus, which is the genus of the present day horse, had very similar teeth. The facial fossa that is characteristic of the “hipparions” is greatly decreased in these two genera. It is believed by Prothero and Schoch (2002) that Dinohippus died out in the middle Pliocene, making the successor Equus the last surviving genus to enter the Pleistocene epoch (Table 4.1). Equus radiated throughout North America and the entire Old World, although did not infiltrate South America.

The number of divisions in the Equus genus is still being debated in scientific circles. From 1842 until the present, over 59 “distinct” species (based on dental variations) of Equus were found in North America (Prothero and Schoch, 2002). However, in the past few years, scientists have begun to accept that variation within a species is normal and that 59 distinct species may be too many. In 1980, only fifteen species were recognized and by 1989, the number had been reduced to five groups. A final definitive number has yet to be reached. Developed in 1983, the following chart (Table 4.3) gives a general idea of the species divisions within Equus.

Table 4.3 -Equus Species

Source: adapted from Sautter and Glover, 1981 and Waring, 1982

Although the terrain of North America was evidently welcoming to the evolving horses throughout the Cenozoic era, all species of Equus mysteriously died out approximately 10,000 years ago. Some have blamed climatic change for the extinction, although other similar mammals were able to survive the Ice Ages. Primitive hunters also are sometimes blamed, although it seems rather implausible that hunters could demolish the millions of herds of horses roaming the world. There is yet to be evidence supporting the theory of massive hunting of equine herds. It is quite ironic that “North America, which had long been the center of horse evolution, today derives all its horses from descendants that left the continent hundreds of thousands of years ago” (Prothero and Schoch, 2002, 215). For whatever reason, horses become extinct and didn’t appear in North America until Christopher Columbus’ voyages in the 1490’s (Prothero and Schoch, 2002).

All domestic and wild horses living today are part of the Equus caballus species. These horses left North America on the Bering land-bridge during the early Pleistocene epoch, before the extinction. In Europe and Asia, Equus caballus diversified, each group adapting to the differing environments. Some horses lived in the forest-grass terrains of east-central Europe, while some adapted to the colder bog country of northern Europe. Others lived in the arid hardgrounds of the deserts around the Mediterranean and Near East. A group of horses living in the tundra along the ice front in Asia, known as Equuscaballus przewalksii (Przewalski’s horse, see Figure 4.4) were the only horses to survive extinction, although several species of zebra and asses also survived. Most scientists now believe that Przewalski’s horse is the stock from which all domesticated horses are descended (Prothero and Schoch, 2002). The last known Przewalski’s horse was caught alive in 1947 in the Desert of Mongolia. In 1978, the Russians searched the deserts for any remaining Przewalski’s horses, but did not find a single horse. They now exist only in captivity and are often used to study natural equine behavior.

Equus caballus was known to have been hunted for meat, as kill sites have been discovered along with cave drawings from Lascaux, France (Figure 4.5). While the exact transition from prey animal to domestic animal is unknown, scientists suspect the process happened independently in several areas around the globe between 4,000 and 6,000 years ago (Prothero and Schoch, 2002 and Sautter and Glover, 1981). Hungarian scientist Sador Bokonyi determined that the domestication of horses first took place in Ukraine, in a settlement called Dereivka around 6,000 years ago (Sautter and Glover, 1981). Several years later, Zeuner’s evidence that wild horses would have been living in this region during this time period confirmed Bokonyi’s argument (Sautter and Glover, 1981). The nomadic people of Dereivka settlement were known to have herded sheep and goats, therefore the transition to horse domestication would not have been difficult. There is also evidence that the peoples in China, Spain and Africa domesticated horses some 4000 years ago, a time when horse bones would have been found in association with human settlement. “Domestication [of horses] was so easy and useful that many cultures stumbled upon it” (Prothero and Schoch, 2002, 226).

Of all domestic animals, horses most likely had the most profound effects on human civilization. Cattle, sheep and goats admittedly provided a supply of food and clothing. However, “the whole pattern of human history was altered by the domestication of the horse” (Sautter and Glover, 1981, 39). Without the domestication of horses, the advancements made through mounted warfare, travel and communication may not have been possible. Horses have been indispensable tools for the developing agricultural communities, both in the past and present. Horses have been symbols of wealth and power not only during Europe’s knighthood and chivalry era, but also in ancient Athens, Rome, France, Spain and Germany. This symbolism is evident throughout the world in the present day, as mounted games such as Thoroughbred racing have become universal signs of wealth and privilege.

Table 4.4 -Evolutionary Ladder

Source: Prothero and Schoch, 2002

[1] Since the work of Prothero and Schoch is most recent (2002), it is now the basis of much of the knowledge of equine evolution and is emphasized in this section.